It is conventional in circuit design to supply circuit board components with relatively low DC voltages from regulated power supplies having low output impedances. Such supply voltages often include low frequency ripple. Additionally, such circuit boards often include components that are highly sensitive to any AC ripple in the supply voltage.
More specifically, exemplary circuit board components, such as phase lock loop circuit modules, although tolerant to a range of constant voltages are intolerant or very sensitive to low frequency ripple included in such constant voltages. Furthermore, attempts to supply such reference voltages that are essentially ripple free through the use of simple RC filters fail to solve the problem since selection of component values are subject to size constraints, and when selected to be sufficiently large as to be within acceptable size limits, either result in an insufficient amount of filtering or cause the input voltage to drop below minimum acceptable values for correct operation of the circuit components being supplied with the reference voltage.
I have discovered a manner of increasing ripple rejection without sustaining unacceptable voltage loss due to loading and thus insure proper operation of ripple sensitive circuits such as phase lock loop circuit modules. Essentially, such beneficial results are obtained by first filtering the reference voltage through the use of a simple RC filter wherein the filter components are selected so as to obtain good ripple rejection. Coupled to this first filter section is a DC coupled emitter follower amplifier stage which due to its characteristic high input impedance does not load down the reference voltage produced by the first section of the filter. Moreover, this active second filter section reproduces at its output a waveform that includes the reduction in ripple obtained by the first filter section. The reduced ripple preserved by the active second stage, however, is produced at a voltage that is increased by the voltage drop normally obtained by way of a forward biased emitter-base junction.
Thus, the output produced at the second stage of the active filter is at a level above the minimum acceptable input reference voltage and is also sufficiently high to allow a second stage of ripple rejection filtering to be included so long as the component values are selected to maintain at least the minimum acceptable reference voltage required by the circuit components being supplied.
The exemplary embodiment of the disclosed invention additionally includes a circuit diode for allowing fast charge of the filter components when the device is first turned on. Such inclusion permits downstream components to be completely operative with little or no time loss or delay.
Although the active lowpass ripple filter disclosed herein was conceived as a solution to a particular problem, namely, that of supplying a ripple free voltage to a phase lock loop circuit module on the transmit-receive circuit board of a portable radio, it is believed that those skilled in the art will recognize that my solution is applicable to a general class of problems involving the supply of ripple free reference voltages. Moreover, the objects and advantages of my invention will be more completely understood and appreciated by the artisan carefully studying the following description of the presently preferred exemplary embodiment taken in conjunction with the accompanying drawings of which: